From: Leif Middelschulte <leif.middelschulte@gmail.com>
To: "Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>,
"Shawn Guo" <shawnguo@kernel.org>,
"Sascha Hauer" <s.hauer@pengutronix.de>,
"Pengutronix Kernel Team" <kernel@pengutronix.de>,
"Fabio Estevam" <festevam@gmail.com>,
"NXP Linux Team" <linux-imx@nxp.com>
Cc: Leif Middelschulte <Leif.Middelschulte@klsmartin.com>,
linux-pwm@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
linux-kernel@vger.kernel.org
Subject: [PATCH v4 1/2] pwm: imx27: fix race condition .apply,.get_state
Date: Sat, 24 Feb 2024 12:29:00 +0100 [thread overview]
Message-ID: <20240224112902.55539-1-Leif.Middelschulte@gmail.com> (raw)
In-Reply-To: <20230906154215.4ikrrbx4xgx2nmu5@pengutronix.de>
From: Leif Middelschulte <Leif.Middelschulte@klsmartin.com>
With CONFIG_PWM_DEBUG=y after writing a value to the PWMSAR
register in .apply(), the register is read in .get_state().
Unless a period completed in the meantime, this read yields the
previously used duty cycle configuration. As the PWM_DEBUG code
applies the read out configuration for testing purposes this
effectively undoes the intended effect by rewriting the previous
hardware state.
Note that this change merely implements a sensible heuristic.
The i.MX has a 4 slot FIFO to configure the duty cycle. This FIFO
cannot be read back in its entirety. The "write x then read back
x from hw" semantics are therefore not easily applicable.
With this change, the .get_state() function tries to wait for some
stabilization in the FIFO (empty state). In this state it keeps
applying the last value written to the sample register.
Signed-off-by: Leif Middelschulte <Leif.Middelschulte@klsmartin.com>
---
drivers/pwm/pwm-imx27.c | 55 +++++++++++++++++++++++++++++++++++++++--
1 file changed, 53 insertions(+), 2 deletions(-)
diff --git a/drivers/pwm/pwm-imx27.c b/drivers/pwm/pwm-imx27.c
index 7d9bc43f12b0..cb564460b79c 100644
--- a/drivers/pwm/pwm-imx27.c
+++ b/drivers/pwm/pwm-imx27.c
@@ -75,6 +75,7 @@
(x)) + 1)
#define MX3_PWM_SWR_LOOP 5
+#define MX3_PWM_FIFOAV_EMPTY_LOOP 4
/* PWMPR register value of 0xffff has the same effect as 0xfffe */
#define MX3_PWMPR_MAX 0xfffe
@@ -118,6 +119,31 @@ static void pwm_imx27_clk_disable_unprepare(struct pwm_imx27_chip *imx)
clk_disable_unprepare(imx->clk_ipg);
}
+static int pwm_imx27_wait_fifo_empty(struct pwm_chip *chip,
+ struct pwm_device *pwm)
+{
+ struct pwm_imx27_chip *imx = to_pwm_imx27_chip(chip);
+ struct device *dev = chip->dev;
+ unsigned int period_ms = DIV_ROUND_UP_ULL(pwm->state.period, NSEC_PER_MSEC);
+ int tries = MX3_PWM_FIFOAV_EMPTY_LOOP;
+ int fifoav, previous_fifoav = INT_MAX;
+ u32 sr;
+
+ while (tries--) {
+ sr = readl(imx->mmio_base + MX3_PWMSR);
+ fifoav = FIELD_GET(MX3_PWMSR_FIFOAV, sr);
+ if (fifoav == MX3_PWMSR_FIFOAV_EMPTY)
+ return 0;
+ /* if the FIFO value does not decrease, there is another problem */
+ if (previous_fifoav == fifoav)
+ break;
+ previous_fifoav = fifoav;
+ msleep(period_ms);
+ }
+
+ return -EAGAIN;
+}
+
static int pwm_imx27_get_state(struct pwm_chip *chip,
struct pwm_device *pwm, struct pwm_state *state)
{
@@ -161,10 +187,35 @@ static int pwm_imx27_get_state(struct pwm_chip *chip,
* PWMSAR can be read only if PWM is enabled. If the PWM is disabled,
* use the cached value.
*/
- if (state->enabled)
+ if (state->enabled) {
+ /*
+ * From the i.MX PWM reference manual:
+ * "A read on the sample register yields the current FIFO value that
+ * is being used, or will be used, by the PWM for generation on the
+ * output signal. Therefore, a write and a subsequent read on the
+ * sample register may result in different values being obtained."
+ * Furthermore:
+ * "When a new value is written, the duty cycle changes after the
+ * current period is over."
+ * Note "changes" vs. "changes to the given value"!
+ * Finally:
+ * "The PWM will run at the last set duty-cycle setting if all the
+ * values of the FIFO has been utilized, until the FIFO is reloaded
+ * or the PWM is disabled."
+ * Try to be at least a bit more deterministic about which value is
+ * read by waiting until the FIFO is empty. In this state the last/most
+ * recently pushed sample (duty cycle) value is continuously applied.
+ * Beware that this approach is still racy, as a new value could have
+ * been supplied and a period expired between the call of the wait
+ * function and the subsequent readl.
+ */
+ ret = pwm_imx27_wait_fifo_empty(chip, pwm);
+ if (ret)
+ return ret;
val = readl(imx->mmio_base + MX3_PWMSAR);
- else
+ } else {
val = imx->duty_cycle;
+ }
tmp = NSEC_PER_SEC * (u64)(val) * prescaler;
state->duty_cycle = DIV_ROUND_UP_ULL(tmp, pwm_clk);
--
2.39.3 (Apple Git-145)
next prev parent reply other threads:[~2024-02-24 11:29 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-01-26 15:03 [PATCH v2] pwm: imx27: fix race condition .apply,.get_state Leif Middelschulte
2023-03-10 17:45 ` Uwe Kleine-König
2023-08-15 10:43 ` [PATCH v3 1/4] " Leif Middelschulte
2023-08-15 10:43 ` [PATCH v3 2/4] pwm: imx27: avoid PWM consumer API Leif Middelschulte
2023-09-06 15:36 ` Uwe Kleine-König
2023-08-15 10:43 ` [PATCH v3 3/4] pwm: imx27: verify decreasing PWM FIFO av value Leif Middelschulte
2023-08-15 10:43 ` [PATCH v3 4/4] pwm: imx27: return error, if clean PWM setup fails Leif Middelschulte
2023-09-06 15:44 ` Uwe Kleine-König
2023-09-06 15:42 ` [PATCH v3 1/4] pwm: imx27: fix race condition .apply,.get_state Uwe Kleine-König
2024-02-24 11:00 ` Leif Middelschulte
2024-02-24 11:29 ` Leif Middelschulte [this message]
2024-02-24 11:29 ` [PATCH v4 2/2] pwm: imx27: avoid PWM consumer API Leif Middelschulte
2024-02-26 8:37 ` Uwe Kleine-König
2024-02-26 8:24 ` [PATCH v4 1/2] pwm: imx27: fix race condition .apply,.get_state Uwe Kleine-König
2024-02-26 8:44 ` Uwe Kleine-König
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